When I finished my game and started digging into people's experiences, I found we were in a definite minority that we opened package 8. We did it early-ish - maybe in the spring. Those three cards were one of the most fun parts of the entire season. They were powerful and we didn't want to waste them, but we also didn't want to lose a game when they could've made the difference. They led to a lot of discussion, arguing, and we even created a protocol to use one - all four players had to grab one corner each and tear to use it, so everyone had to be on board. Obviously you lose a lot of points because you have to open it, but to me, it was an interesting aspect. Maybe if it had a larger penalty - we only ever didn't use it because we didn't want to lose it, and one game we used it and lost anyway - it could've been thrown in the mix, but I'm glad we ended up having that aspect of the game, and I feel like by making it a handicapping mechanism, some groups may have missed out.

Well, never during actual play, anyway. We peeked once we'd finished. (-8

For me, the most interesting trade-off is that while those cards remain in your deck they're giving you an extra 1.5 turns per game. Is that benefit worth losing?

The second most interesting is that cancelling an Epidemic has an non-obvious effect on the infection deck. You're going to see a much more diverse range of cities than usual, and have a much larger intensification than usual next time.

(Now I think about it, Package 8 also makes it possible for the infection deck to run out. I wonder what one is supposed to do if that happens!)

Almost. Season 2 also says to immediately increase the Infection Rate by 1.

As far as I can see, Season 1 says nothing. Probably the designers didn't consider it, because it's the sort of thing that normally never happens in vanilla Pandemic (I don't know if it can; I know that, personally at least, I've never actually seen it, even when we used to regularly play with only 4 epidemics).